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semiconducting polymers are expected to have a great contribu-
tion to organic electronics in the near future because it will be
possible to manufacture light emitting diodes, bipolar transistors,
and the polymeric analog of silicon field-effect transistors [9].
Thus, conducting polymers with stable negatively doped states
have drawn great attention in the field of electrochromism.
Following this strategy, we previously reported the synthesis of
poly[2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(3,4-ethylenedioxy-
thiophene)) quinoxaline] (PMFEQ), poly[2,3-di(5-methylfuran-2-
yl)-5,8-bis(2-thienyl) quinoxaline] (PMFTQ) and poly[2,3-di(5-
methylfuran-2-yl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline]
(PMFMQ) containing the strong electron-accepting 2,3-di(5-
methylfuran-2-yl) quinoxaline as the acceptor unit. The electro-
chemical band-gaps of these three polymers have been reported to
be between 0.90 and 1.20 eV, and all of them present significant n-
type doping [10].
electrode was an ITO/glass electrode, the counter electrode was a
stainless steel wire, and a Ag wire (0.03 V vs. SCE.) was used as a
pseudo-reference electrode. The polymer films for spectroelec-
trochemistry were prepared by potentiostatic deposition on an
ITO/glass electrode with an active area of 0.9 cm ꢀ 2.0 cm. The
thickness of the polymer films grown potentiostatically on the ITO/
glass was controlled by the total charge passed through the cell.
Colorimetry measurements were obtained by a SP 60 spectropho-
tometer (X-Rite, USA) with illuminator D65 used as the simulated
light source and CIE 10ꢁ as the illuminating/viewing geometry.
Digital photographs of the polymer films were taken by a Canon
Power Shot A3000 IS digital camera.
2.2. Synthesis
2.2.1. 2,3-Bis(2-furyl)-5,8-dibromoquinoxaline (6)
Considering the contribution of the steric interaction between
the repeating units within the polymer backbone, polymers with
more-planar geometries exhibit several special properties and
advantages, including lower band gaps, stronger intermolecular
interactions, and greater degrees of doping [11]. Thus, the 2,3-di(2-
furyl) quinoxaline moiety with a stronger electron-accepting
ability and a greater degree of coplanarity was used as a substitute
for the previous acceptor unit to combine with different thiophene
donors. As a result, three new polymers were electrochemically
synthesized. These were poly[2,3-di(2-furyl)-5,8-bis(2-(3,4-ethyl-
enedioxythiophene)) quinoxaline] (PFETQ), poly[2,3-di(2-furyl)-
5,8-bis(2-thienyl) quinoxaline] (PFTQ) and poly[2,3-di(2-furyl)-
5,8-bis(2-(3-methoxythiophene)) quinoxaline] (PFMTQ). The
amount of decrease in the band gap and the bathochromic shift
of the maximum absorption wavelength of the newly synthesized
polymers containing 2,3-di(2-furyl) quinoxaline demonstrated a
definite improvement in the electrochemical and optical proper-
ties in contrast to the previously reported ones. In addition, it is
worth noting that both PFETQ and PFMTQ showed a green color in
the neutral state and a high transmissivity in the oxidized state.
The effect of the different thiophene derivatives on the photo-
electrical properties was further investigated in detail in this
article.
To a mixture of 3,6-dibromo-1,2-phenylenediamine (1.33 g,
5 mmol) and 1,2-di(2-furyl) ethanedione (0.95 g, 5 mmol) in EtOH
(50 mL), a catalytic amount of p-toluene sulfonic acid (PTSA) was
added, and the solution was then refluxed while being magneti-
cally stirred overnight. At the end of the reaction, a cloudy mixture
was observed. The solution was cooled to 0 ꢁC and filtered. The
separated solid was washed with EtOH several times and dried in a
vacuum oven to give a yellow-green powder (1.8 g, 85.7%). 1H NMR
(CDCl3, 400 MHz, ppm):
d= 7.88 (s, 2H, ArH), 7.63 (d, 2H), 6.97
(d, 2H), 6.60 (q, 2H). (See Supporting Information Fig. S1).
2.2.2. General procedure for the synthesis of FTQ,FETQ, FMTQ
2,3-Bis(2-furyl)-5,8-dibromoquinoxaline (1.68 g, 4 mmol) was
subjected to the Stille coupling reaction with excess tributyl-
stannane (16 mmol) in anhydrous THF (60 mL) using Pd(PPh3)2Cl2
(0.28 g, 0.4 mmol) as the catalyst. After the reaction mixture was
deaerated, the temperature was immediately raised to reflux
temperature. The solution was refluxed with stirring under
nitrogen atmosphere for 24 h, cooled and concentrated on the
rotary evaporator. Lastly the residue was purified by column
chromatography on silica gel using hexane- dichloromethane as
the eluent.
2.2.2.1. 2,3-Di(2-furyl)-5,8-bis(2-thienyl) quinoxaline (FTQ). The
crude mixture was chromatographed on silica gel by eluting
with hexane: dichloromethane (15:1, v/v) to give FTQ as an orange
2. Experimental
2.1. General
solid (1.3 g, 76.5%). 1H NMR(CDCl3, 400 MHz, ppm):
d
= 8.10 (s, 2H,
ArH), 7.90 (d, 2H), 7.62 (d, 2H), 7.54 (d, 2H), 7.20 (q, 2H), 7.12 (d, 2H),
6.63 (q, 2H). 13C NMR (CDCl3, 101 MHz, ppm):
= 148.35, 144.32,
All chemicals were purchased from commercial sources and
used without further purification except for tetrahydrofuran which
was dried and distilled over benzophenone and sodium under
nitrogen before use. The compounds 3,6-dibromo-1,2-phenyl-
enediamine (2), [12] 2-hydroxy-1,2-di(2-furyl) ethanone (4), [13]
1,2-di(2-furyl) ethanedione (5) [14] and tributylstannane com-
pounds [15] were prepared according to the methods described in
the literature. The 1H NMR and 13C NMR spectra of the monomers
were recorded on a Varian AMX 400 spectrometer in CDCl3 at
d
134.85, 133.08, 131.45, 125.74, 123.58, 121.61, 121.59, 119.35, 110.31,
104.43. (See Supporting Information Fig. S2) HRMS (m/z, EI+) calcd
for C24H14N2O2S2 426.5066, found 426.5058.
2.2.2.2.
2,3-Di(2-furyl)-5,8-bis(2-(3,4-ethylenedioxythiophene))
quinoxaline (FETQ). The crude mixture was chromatographed
on silica gel by eluting with hexane: dichloromethane (3:1, v/v) to
give FETQ as a deep red solid (1.6 g, 73.8%). 1H NMR (CDCl3,
400 MHz, and chemical shifts (
d
) were reported relative to
400 MHz, ppm):
(q, 2H), 6.57 (s, 2H), 4.33 (dd, 8H). 13C NMR (CDCl3, 101 MHz, ppm):
= 151.62, 143.95, 141.33, 140.42, 139.35, 136.54, 128.48, 128.16,
113.67, 113.17, 111.90, 103.28, 64.91,64.28. (See Supporting
Information Fig. S3) HRMS (m/z, EI+) calcd for C28H18N2O6S2
542.5782, found 542.5810.
d= 8.58 (s, 2H, ArH), 7.58 (d, 2H), 7.12 (d, 2H), 6.60
tetramethylsilane as the internal standard. Electrochemical
behaviors were investigated by cyclic voltammetry (CV). Electro-
chemical synthesis and experiments were performed in a one-
compartment cell with a CHI 760 C Electrochemical Analyzer under
the control of a computer, employing a platinum wire with a
diameter of 0.5 mm as a working electrode, a platinum ring as a
counter electrode, and a Ag wire (0.03 V vs. SCE.) as a pseudo-
reference electrode. Scanning electron microscopy (SEM) meas-
urements were taken by using a Hitachi SU-70 thermionic field
emission SEM. UV–Vis–NIR spectra were recorded on a Varian Cary
5000 spectrophotometer connected to a computer. A three-
electrode cell assembly was included in which the working
d
2.2.2.3. 2,3-Di(2-furyl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline
(FMTQ). The crude mixture was chromatographed on silica gel by
eluting with hexane: dichloromethane (8:1, v/v) to give FMTQ as a
bright red solid (1.5 g, 77.2%). 1H NMR (CDCl3, 400 MHz, ppm):
d
= 8.02 (s, 2H, ArH), 7.62 (d, 2H), 7.59 (d, 2H), 7.10 (d, 2H), 6.63
(d, 2H), 6.48 (q, 2H). 3.88 (s, 6H). 13C NMR (CDCl3, 101 MHz, ppm):